Fibre optical cables have been used for many years to transmit information. The fibre optical cables includes hair-thin optical fibres as the transmission medium. These optical fibres are protected by core or buffer tube structures from external forces and elements. Depending on the construction, the fibre optic cables may be divided in four different categories. These are the slotted core type, the tight-coated tube type, the stranded-loose tube type, and the central tube type. Some of these categories can be subdivided further depending on how the fibres are inserted into the core. Thus, the fibres may be inserted as individual fibres, fibre ribbons, or fibre bundles. The most frequently used type of fibre optic cable today is the stranded-loose tube type, but an increasing interest has been shown in the other types, especially the slotted core type which is of particular significance in connection with the present invention.
As an example of fibre optic cables reference may be made to U.S. Pat. No. 5,574,816 which discloses buffer tubes for optical fibre cables.
In the stranded-loose tube type cable the optical fibres reside in protecting and supporting elements in the form of gel filled buffer tubes. These buffer tubes are stranded around a central strengthening member.
In a fibre optical cable of the slotted core type, the optical fibres reside in gel filled channels or slots in the surface of a so-called slotted core supporting element. The core has the form of a central strengthening member of steel or fibre glass reinforced plastic around which is extruded a plastic rod with circumferential slots or groves in the surface. These slots of the slotted core are symmetrical and form a winding helical or SZ like path along the longitudinal axis of the cable. The optical fibres inserted into the slots may be loose fibres or fibre ribbons. The slotted core with its optical fibre and gel filled slots is secured with a binding tape and covered by an outer sheathing. For further information regarding fibre optical cables of the slotted core type reference may be made to an article by Mikko Saikkonen, "Extrusion of slotted core elements", Wire Technology International, November 1995.
The supporting elements, such as the slotted core elements for slotted core type cables and the buffer tubes of the stranded loose tube type cables are made to very exacting requirements. This puts very severe requirements on the materials used for these elements. Normally, the elements are made of polymer materials by melt extrusion. For easy production the polymer material should have a good melt processability; to cope with the mechanical stresses it is exposed to in use it should have good mechanical properties such as a sufficient strength in the solid state; and, very important particularly to the slotted core type cable, the polymer material should have a good dimensional stability to keep the profile shape stable, i.e. so that the ridges of the slotted core are not deformed or collapses. Another important property is the melt strength of the material, and further, the polymer material should have a low shrinkage. Many of these requirements are opposed to each other, i.e. if one property of the polymer material is optimized another property is likely to be deleteriously affected. The polymer materials used hitherto which are mostly unimodal polyethylene polymers therefore tend to be compromises with regard to their properties.
There is an ongoing need in the art for improved polymer materials for fibre optical cables, and more particularly for supporting elements such as slotted cores and buffer tubes. Specifically there is a need for a polymer material with improved dimensional stability and the need for this is particularly strong with regard to slotted cores.